The object of the present invention is to provide a magenta-colored 2,9-dimethylquinacridone pigment of high color strength, excellent dispersability, high heat stability and high saturation. This object is achieved by resinating 2,9-dimethylquinacridone with a mixture of acid salts containing a resin acid salt and a naphthenic acid salt or dimeric acid salt, preferably in the presence of a pigment particle growth inhibitor.
2,9-dimethylquinacridone is a valuable organic pigment. Due to its unique magenta shade, it is used in large quantities for the pigmentation of plastics, inks and coatings in the worldwide market. It is particularly useful as a shading component in combination with other pigments.
The resination of quinacridone pigments to improve their coloristic and dispersibility properties is also known. In general, resinated pigments are prepared by co-milling the crude pigment with the resin, such as a resin acid.
The present invention is based on the discovery that 2,9-dimethylquinacridone, which is mixed with a small amount of a pigment particle growth inhibitor, and resinated with a mixture of a resin acid salt and a naphthenic acid salt or a dimeric acid salt, or a mixture thereof, surprisingly possesses improved properties relative to non-resinated 2,9-dimethylquinacridone and higher heat stability relative to 2,9-dimethylquinacridone which is resinated with the resin acid salt alone.
Thus, the present invention relates to an improved resinated 2,9-dimethylquinacridone and a process for its preparation. The resinated 2,9-dimethylquinacridone possesses excellent dispersability, high color strength, high heat stability, high weatherability and remarkable saturation. It is prepared by resinating 2,9-dimethylquinacridone in a suspension containing the 2,9-dimethylquinacridone and a mixture of a water-soluble resin acid salt and a water-soluble naphthenic and/or dimeric acid salt and precipitating the resin acid and naphthenic and/or dimeric acid by forming an insoluble salt, wherein a pigment particle growth inhibitor is added to the suspension before, during or after the the mixture of acid salts is precipitated. The resinated pigment is suitable for coloring plastics, inks and coatings, especially applications which require excellent heat stability and/or high saturation.